(1) Field of the Invention
The present invention relates to an easy-open can lid. More particularly, the present invention relates to an easy-open can lid formed from a chromated surface treated steel plate, which is excellent in easy openability, corrosion resistance and resistance against compression deformation and which can be easily manufactured and is suitable for reclamation after use.
(2) Description of the Prior Art
An easy-open lid comprising a portion to be opened, which is defined by a score line (partially cut line), a rivet formed on this portion to be opened and a pulling tab secured to this rivet is widely used for a can for containing therein a drink such as cola, beer or juice. In this easy-open can lid, by pulling the tab, the portion to be opened is cut and taken out along the score line.
This easy-open can lid of the score-breakage type is excellent in the combination of sealing reliability and easy openability, but it still involves problems to be solved. More specifically, in the manufacture of a can lid of this type, severe processings such as scoring processing and riveting processing should be performed, and therefore, the material that can be used is limited to a material excellent in the processability, such as aluminum. Accordingly, even when a can body is composed of a surface-treated steel plate such as tin-free steel (TFS), an easy-open lid has to be formed of aluminum. From the viewpoint of prevention of can pollution or saving of resources, it is desirable to recover used can bodies and reclaim them. However, since lids are composed of a material different from that of can bodies, it is extremely difficult to reclaim used can bodies.
Moreover, an inner coating of a can lid is easily damaged by severe processing such as riveting processing, and therefore, when a corrosive content is filled and a severe treatment such as heat sterilization is carried out as in case of canned food, no satisfactory corrosion resistance can be obtained. This tendency is especially conspicuous in case of an easy-open can lid formed by using aluminum. Furthermore, when a content having a high sodium chloride concentration is filled, pitting corrosion is caused and problems such as leakage of the content or contamination with bacteria through piercing holes arise.
As means for obviating these problems involved in an aluminum can lid, there has been proposed an easy-open lid composed of a steel plate. However, in the steel lid, a force required for opening is larger than in an aluminum lid and opening of the lid is generally difficult. As means for reducing the opening force in a steel lid, a method in which a carbide powder having an average particle size of 2.0 to 8.0 .mu.m is precipitated in the steel plate and voids acting as the stress-concentrating source are formed between base iron and carbide particles is proposed, for example, in Japanese Patent Publication No. 61815/82. Indeed, according to this proposal, the opening force is reduced, but the steel plate per se becomes brittle and the portion to be opened is separated from the lid at a stretch, and the hand or finger is readily hurt by the cut edge.